1. Field of Invention
Embodiments of the present invention relate generally to a processing system and a method for polishing a substrate.
2. Background of Invention
In semiconductor wafer processing, the use of chemical mechanical planarization, or CMP, has gained favor due to the enhanced ability to increase device density on a semiconductor workpiece, or substrate, such as a wafer. As the demand for planarization of layers formed on wafers in semiconductor fabrication increases, the requirement for greater system (i.e., process tool) throughput with less wafer damage and enhanced wafer planarization has also increased.
Two exemplary CMP systems that address these issues are described in U.S. Pat. No. 5,804,507, issued Sep. 8, 1998 to Perlov et al. and in U.S. Pat. No. 5,738,574, issued Apr. 15, 1998 to Tolles et al., both of which are incorporated by reference. Perlov et al. and Tolles et al. disclose a CMP system having a planarization system that is supplied wafers from cassettes located in an adjacent liquid filled bath. A transfer mechanism, or robot, facilitates the transfer of the wafers from the bath to a transfer station. The transfer station generally contains a load cup that positions wafers into one of four processing heads mounted to a carousel. The carousel moves each processing head sequentially over the load cup to receive a wafer. As the processing heads are loaded, the carousel moves the processing heads and wafers through the planarization stations for polishing. The wafers are planarized by moving the wafer relative to a polishing material or pad in the presence of a slurry or other polishing fluid medium. The polishing pad may include an abrasive surface. The slurry typically contains both chemicals and abrasives that aid in the removal of material from the wafer. After completion of the planarization process, the wafer is returned back through the transfer station to the proper cassette located in the bath.
Conventional polishing pads are generally comprised of a foamed polymer having a textured or porous surface. The textured or porous surface functions to retain the polishing fluid on the polishing pad during the polishing operation. For example, during the polishing motion of the substrate relative to the polishing pad, the polishing fluid may be swept or spun off of the surface of the polishing pad, or otherwise become non-uniform between the polishing pad and substrate. The non-uniformity of polishing fluid between the polishing pad and substrate, such as less fluid or fluid component (chemical or abrasive) in one location as opposed to another location, may lead to a non-uniform rate of material removal (e.g., poor planarization) from the substrate.
Over the course of processing a number of wafers, the texture or pores on the polishing surface may become glazed over with polishing byproducts or deformed by forces applied to the polishing surface during polishing. For example, pores residing in the polishing surface generally are orientated normal to the polishing surface, and have a certain open area. As the forces applied to the polishing surface during polishing deform the polishing surface, the open area of the pores become increasingly smaller or have their openings on the polishing surface closed off. Reduced area pores and closed pores have a diminished polishing fluid retaining capacity. As the pores are continued to be pushed one side after repeated polishing operations, the pores in the polishing surface may no longer be able to properly (i.e., uniformly) retain the polishing fluid during polishing. In order to return the polishing surface to a condition that supports a uniform rate of planarization, the polishing pad must be conditioned. For example, when polishing tungsten, pores quickly become closed and may require conditioning of the polishing surface after polishing between 100 to 300 wafers.
Conditioning is generally performed by using a silicon carbide or other hard material or textured element which may be placed against the polishing pad to dress (e.g., return) the polishing pad to a state where the polishing fluid is adequately retained. Generally, the conditioning operation must be performed periodically in order to maintain uniform planarization from wafer to wafer and even within a wafer. Since no wafers may be processed during the conditioning operation, valuable production time and product capacity is lost during each conditioning operation.
Therefore, there is a need in the art for a apparatus and process that reduces the need to condition the polishing pad.